Recent communication devices, e.g. a mobile phone or a smart phone, are designed to provide high data throughput for inbound and outbound RF signals.
A method called carrier aggregation is used for increasing the data throughput by increasing the bandwidth of the inbound signals (downlink) and of the outbound signals (uplink). Carrier aggregation, which is part of the 3GPP UMTS standard and the LTE standard enables utilization of more than one carrier (or cell in UMTS terminology) for transmission and reception. Each aggregated carrier is referred to as a component carrier (CC). The component carrier may have a bandwidth of 1.4, 3, 5, 10, 15 or 20 MHz and a maximum of five component carriers can be aggregated, hence the maximum aggregated bandwidth is 100 MHz.
The distinction may be made between intra-band contiguous aggregation, intra-band non-contiguous aggregation and inter-band aggregation (aggregating of spectra being in different bands). Below, the handling of multiple (aggregated) carriers within a transmitter (e.g. a transmitter of a mobile communication device) will be discussed, wherein the aggregation in case of a receiver is based on the same principle and may have comparable characteristics.
In a transmitter having a plurality of transmit chains, for example two, a separate transmit chain is used for each carrier of the aggregated carriers. Here, each transmit chain just needs to support the transmission bandwidth of the one (component) carrier. However, the use of separate transmit chains causes additional chip space requirements. Furthermore, more than one, e.g. two, LO signals are needed to transfer the baseband signals to the RF domain. These LO signals may be very close in frequency, especially in an intra-band contiguous case, and therefore the unit generating these LO signals, e.g. a PLL (phase locked loop), is very sensitive with respect to cross-talk. Therefore, there is a need for an improved approach.